Multi-hop matters: the state of wireless mesh networking

After many years of hype, multi-hop mesh networking has finally become an …

Multi-hop mesh networks, confined to university labs at the start of this decade, are now widely available from commercial vendors. These vendors tout a number of advantages for mesh technologies: lower costs of deployment, easier administration, better coverage, and lower power consumption. Mesh networking is now being used in an impressive range of applications, from large-scale institutional deployments to networks of tiny sensors.

Mesh networking is sometimes mentioned as a solution to the much-discussed "last mile" problem in US telecommunications policy. Unfortunately, the inherent capacity limits of the wireless medium means that mesh networks are unlikely to provide a serious alternative to fiber or coax broadband connections in this market. Mesh is a reasonable way to provide broadband to consumers in developing countries who might not otherwise be able to afford access at all. But in the developed world, mesh technologies are best viewed as a supplement to wired Internet connections and traditional single-hop access points.

To help us understand the state of technology, Ars talked to two experts on mesh networking: Sanjit Biswas, the CEO of mesh wireless startup Meraki Networks, and Jinyang Li, a computer science professor at NYU. Each did pioneering work on mesh networks at MIT earlier this decade, and both continue working on mesh networking technologies today.

Both identified the relative paucity of spectrum available for mesh networking applications as a major constraint on the technology's continued growth. Most mesh networks today operate on the same unlicensed bands used by WiFi and other consumer electronics devices. The amount of unlicensed spectrum available has increased with the deregulation of the 5GHz band, but the vast majority of the spectrum is locked up for other uses.

The evolution of mesh

Computer scientists have been studying mesh networks for a quarter century. In the 1980s, the military funded research into self-configuring multi-hop wireless networks suitable for use on the battlefield, but a lack of affordable hardware limited researchers' ability to build working systems. In the 1990s, researchers began building and testing mesh networks. In one influential test in 1998, a team at Carnegie Mellon mounted computers and networking gear into five cars and drove them around campus. The car-mounted nodes were able to automatically adapt to changes in network topology, maintaining reliable connectivity among themselves and providing continuous connectivity between two fixed nodes at opposite ends of campus.

By the turn of the century, wireless networking gear had become cheap and powerful enough to allow more ambitious tests. One of the most famous is Roofnet, an experimental mesh network built by MIT researchers earlier this decade, and Meraki, the commercial spin-off that counts Google among its investors. Sanjit Biswas co-led the Roofnet team and co-founded its commercial spin-off Meraki.

In 2001, Biswas was a grad student at MIT exploring ways to provide wireless broadband access to residents near the MIT campus in Cambridge. An initial plan for a single large antenna was scrapped when it became clear that it would not provide sufficient range or throughput. Biswas and his fellow grad student John Bicket switched to a multi-hop design, with multiple nodes cooperating to cover a large area.

When fully operational, Roofnet covered about 5 square miles of Cambridge, MA, between the MIT and Harvard campuses. At its peak, it had almost 100 nodes, and Biswas estimates the network served thousands of MIT and Harvard students over its lifetime. Each Roofnet node was an ordinary PC with a WiFi card and antenna. Because the project relied on volunteers to deploy Roofnet nodes in their homes, the Roofnet team had no real control over the topology of the network.

"We had to understand what it takes to build a network that can survive a lot of failures," Biswas said. "People would move from their apartments, or trees would go up in the springtime, blocking the signal." The network's routing protocols had to be robust enough to handle a network with constantly-changing topology and disappearing nodes.

The Roofnet team found that traditional routing protocols designed for wired networks didn't work well in the wireless setting. For example, Biswas told Ars that before Roofnet started its research, "It was widely believed that wireless links either worked or they didn't." In reality, there is "a continuous range of packet-delivery rates, all the way from 0 to 100 percent." This means that nodes cannot assume that any reachable node is a reliable node. Instead, mesh routing protocols need to constantly monitor link reliability and avoid routes with high rates of dropped packets.

Mesh routing protocols also have to worry about interference issues that don't crop up in wired networks. This can be a serious obstacle to high throughput for single-radio mesh nodes, because it prevents adjacent nodes in a route from broadcasting simultaneously. The Roofnet team found that in mesh networks with a single radio per node, this means that a two-hop route has less than half the throughput of a single-hop route. Some recent mesh products deal with this problem by having two or three radios in each node. This allows adjacent nodes on a route to transmit on different frequencies, dramatically improving throughput.

Meraki

In 2006, with several years of experience running a full-scale mesh network under their belts, Biswas and Bicket founded Meraki to commercialize their technology. The firm attracted an early investment from Google, and closed a $20 million venture capital round in 2008.

The most lucrative segment of Meraki's mesh networking business is sales to large institutions. Leading hardware vendors such as Cisco have long sold gear designed to blanket a campus, apartment building, or office tower with WiFi coverage. But Biswas argues that the sophisticated meshing technology in its routers makes them easier to configure and manage than products offered by established vendors. Meraki's access points self-configure and then phone home to Meraki servers for further instructions; customers tweak their networks using a Web-based interface hosted on Meraki's servers.

At the low end of Meraki's product line are mesh nodes optimized for providing basic connectivity for areas that lack traditional wired connectivity. These products are popular internationally; Biswas touts customers in 143 countries worldwide. Mesh networks are especially appealing in nations that lack the resources to put extensive fiber or copper networks in the ground. In these areas, a mesh network can extend connectivity to homes and businesses that would not be able to access it in other ways. The XO laptop distributed by the One Laptop Per Child project uses mesh networking protocols for similar reasons.

For remote areas of developing nations, omnidirectional mesh networks must be paired with backhaul connections that use directional antennas for increased range. For example, Lakshminarayanan Subramanian, a computer science professor at NYU, has used WiFi and directional antennas to transmit 7-10 Mbps over a distance of 100 km. These links will never be as fast as a fiber connection, but for poor villages who otherwise wouldn't be able to afford an Internet connection, mesh networking is a boon.

Going the "last mile"

Mesh networks may provide a cheap way to get villages in the developing world online, but they do not seem likely to pose a serious threat to telecom incumbents in the developed world. One of Meraki's leading competitors, Tropos networks, focuses on public-sector clients such as police departments, public utilities, and municipalities wishing to build city-wide wireless networks. One case study on the Tropos website touts the experience of Chaska, MN, which hired Tropos in 2004 to build a municipal WiFi network for the city's 23,000 residents at a cost of $1.3 million.

Tropos says its 802.11 b/g-based network in Chaska offered "1+ Mbps" connectivity. Biswas says Meraki's newer 802.11n-based products boast typical speeds on the order of 10 Mbps. But with Verizon and Comcast already offering 50Mbps connections in some areas, mesh networking technology seems unlikely to be a serious threat to cable and telephone incumbents in the developed world.

Biswas told Ars that Meraki's low-end gear is primarily used in the United States to supplement traditional wired network connections. So, for example, a neighborhood association might deploy Meraki routers in parts of the neighborhood that are not one hop away from a wired access point, on the theory that relatively slow access is better than no access at all. But these small-scale applications are best thought of as a complement, rather than an alternative, to incumbent broadband providers.

Timothy B. Lee / Timothy covers tech policy for Ars, with a particular focus on patent and copyright law, privacy, free speech, and open government. His writing has appeared in Slate, Reason, Wired, and the New York Times.